The present invention relates to an air-fuel ratio control device of an internal combustion engine.
As a method of simultaneously reducing the amount of harmful HC, CO and NO.sub.x components in the exhaust gas, a method has been known in which a three way catalytic converter is arranged in the exhaust passage of an engine.
The purifying efficiency of the three way catalyzer becomes maximum when the air-fuel ratio of the mixture fed into the cylinder of an engine becomes equal to the stoichiometric air-fuel ratio. Consequently, in the case wherein a three way catalytic converter is used for purifying the exhaust gas, it is necessary to equalize the air-fuel ratio of the mixture fed into the cylinder to the stoichiometric air-fuel ratio. As an air-fuel ratio control device capable of equalizing the air-fuel ratio of the mixture fed into the cylinder of an engine to the stoichiometric air-fuel ratio, an air-fuel ratio control device has been known in which a main air bleed passage is connected to a main fuel passage which is in communication with the main nozzle of a carburetor, and a slow air bleed passage is connected to a slow fuel passage which is in communication with the slow fuel port of the carburetor. A main electromagnetic control valve for controlling the amount of air bled into the main fuel passage, and a subelectromagnetic control valve for controlling the amount of air bled into the slow fuel passage are arranged in the main air bleed passage and the slow air bleed passage, respectively. An oxygen concentration detector is arranged in the exhaust passage of the engine, and the output signal of the oxygen concentration detector is converted to a control signal by an electronic control unit. The main electromagnetic control valve and the subelectromagnetic control valve are driven by the control signal and, thereby, the amount of air fed into the main fuel passage from the main air bleed passage and fed into the slow fuel passage from the slow air bleed passage are controlled so that the air-fuel ratio of the mixture fed into the cylinder of the engine approaches the stoichiometric air-fuel ratio. In this air-fuel ratio control device, the main electromagnetic control valve and the subelectromagnetic control valve are so constructed that the opening areas thereof are proportional to the potential level of the control signal issued from the electronic control unit and, in addition, since the relationship between the opening area of the main electromagnetic control valve and the potential level of the control signal is the same as the relationship between the opening area of the subelectromagnetic control valve and the potential level of the control signal, the opening area of the main electromagnetic control valve is the same as that of the subelectromagnetic control valve.
As is known to those skilled in the art, in a carburetor, when the opening degree of the throttle valve is small, fuel is fed from the slow fuel port, but fuel is not fed from the main nozzle. Contrary to this, when the opening degree of when fuel is fed from only the slow fuel port, that is, when the opening degree of the throttle valve is small, is larger than the opening area of the main electromagnetic control valve, which is necessary to equalize the air-fuel ratio of the mixture to the stoichiometric air-fuel ratio when fuel is mainly fed from the main nozzle, that is, when the opening degree of the throttle valve is large. Consequently, in the case wherein the main electromagnetic control valve and the subelectromagnetic control valve are so constructed that the opening area of the main electromagnetic control valve is always equal to the opening area of the subelectromagnetic control valve, as in a conventional air-fuel ratio control device, when, for example, the throttle valve is abruptly opened from a small opening degree to a great extent, the main electromagnetic control valve has an opening area which is larger than the opening area necessary to equalize the air-fuel ratio of the mixture to the stoichiometric air-fuel ratio. As a result of this, during the time the main electromagnetic control valve is set to an opening degree which is necessary to equalize the air-fuel ratio of the mixture to the stoichiometric air-fuel ratio, the mixture fed into the cylinder of the engine becomes lean. Therefore, since a satisfactory high output power of the engine cannot be obtained, a problem occurs in that a good accelerating operation of the engine cannot be obtained.
An object of the present invention is to provide an air-fuel ratio control device capable of obtaining a good accelerating operation of an engine by preventing the mixture fed into the cylinder of the engine from temporarily becoming lean when the throttle valve is opened.
According to the present invention, there is provided an air-fuel ratio control device of an internal combustion engine having at least one cylinder, an intake passage and an exhaust passage, said device comprising: a carburetor arranged in the intake passage and having a float chamber, a main nozzle, a main fuel passage connecting said float chamber to said main nozzle, a main air bleed passage connecting said main fuel passage to the atmosphere, a slow fuel port, a slow fuel passage connecting said float chamber to said slow fuel port, a slow air bleed passage connecting said slow fuel passage to the atmosphere; valve means having a first valve arranged in said slow air bleed passage for controlling the flow area of said slow air bleed passage, a second valve arranged in said main air bleed passage for controlling the flow area of said main air bleed passage, and an electromagnetic device common to said first valve and said second valve and simultaneously actuating said first valve and said second valve while maintaining the flow area of said slow air bleed passage at an area which is larger than the flow area of said main air bleed passage; an oxygen concentration detector arranged in the exhaust passage and detecting components of the exhaust gas in the exhaust passage for producing a detecting signal having a potential level which becomes high or low when an air-fuel ratio of the mixture fed into the cylinder becomes smaller or larger than the stoichiometric air-fuel ratio, respectively, and; an electronic control unit operated in response to the detecting signal of said oxygen concentration detector and producing a control signal for operating said electromagnetic device to actuate said first valve and said second valve so that the air-fuel ratio of said mixture becomes equal to the stoichiometric air-fuel ratio.